Field of the Invention
1. The invention relates to the purification of gases in a pressure swing adsorption system. More particularly, it relates to the improvement of product gas recovery in such a system.
2. Description of the Prior Art
The pressure swing adsorption (PSA) process provides a commercially desirable technique for separating and purifying at least one gas component from a feed gas mixture of said gas component and at least one selectively adsorbable component. Adsorption occurs in an adsorbent bed at a higher adsorption pressure, with the selectively adsorbable component thereafter being desorbed by pressure reduction to a lower desorption pressure. The PSA process is commonly employed in multi-bed systems as is indicated by the Wagner patent, U.S. Pat. No. 3,430,418, relating to a system having at least four beds, and by the Fuderer, et al. patent, U.S. Pat. No. 3,986,849, which discloses the use of at least seven adsorbent beds. As is generally known and described in these patents, the PSA process is commonly carried out, on a cyclic basis, in a processing sequence that includes, in each bed, higher pressure adsorption with the release of product effluent from the product end of each bed, cocurrent depressurization to intermediate pressure with release of void space gas from the product end of the bed, countercurrent depressurization to a lower desorption pressure, purge and repressurization. The void space gas released during the cocurrent depressurization step is commonly employed for pressure equalization purposes and to provide purge gas to a bed at its lower desorption pressure.
Multi-bed systems have the inherent advantage of greater productivity coupled with a substantially uniform flow of product effluent therefrom. The necessary cycling of the processing sequence from one bed to another is recognized, however, as creating conditions that somewhat limit the recovery of product from such systems. Such effects become more pronounced as the number of beds and the productivity requirements of a PSA system are increased, resulting in a potential increase in the amount of product gas unrecovered from each bed and subsequently lost upon countercurrent depressurization and purge. Such a loss of product gas, while tolerable in light of the overall objects of particular commercial applications of the PSA process and system, is nevertheless undesired.
One such condition tending to lower product recovery is that limitation on the amount of time available for purging each bed prior to the commencing of repressurization in that bed. When the purge time is relatively short, the purge gas must be passed through the bed under relatively high pressure drop conditions in order to adequately purge the bed within such limited available time. While the desired level of product purity can be maintained under such conditions, a relatively high pressure drop is disadvantageous with respect to product recovery, as more desired product is withdrawn from the system with the purge effluent under such circumstances. Such a limitation on product recovery is, of course, undesired in the art.
It is an object of the invention, therefore, to provide an improved PSA process and system.
It is another object of the invention to provide a PSA process and system having improved product recovery.
It is a further object of the invention to provide a PSA process and system capable of enhancing the time available for purge purposes during each processing cycle.
With these and other objects in mind, the invention is hereinafter described in detail, the novel features thereof being particularly pointed out in the appended claims.